A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive layer such as copper.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned layer that contacts the tunnel barrier. The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) and low coercivity (Hc), a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2.
A MgOx based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for AlOx or TiOx based MTJs. In order to achieve a smaller Hc but still maintain a high TMR ratio, the industry tends to use CoFeB as the free layer in a TMR sensor. Unfortunately, the magnetostriction (λ) of a CoFeB free layer is considerably greater than the maximum acceptable value of about 5×10−6 for high density memory applications. A free layer made of a CoFe/NiFe composite has been employed instead of CoFeB because of its low λ and soft magnetic properties. However, when using a CoFe/NiFe free layer, the TMR ratio will degrade. Thus, an improved free layer in a TMR sensor is needed that provides low magnetostriction in combination with a high TMR ratio, low RA value, and low coercivity.